Semantic analysis. Compiler Construction. An expression evaluator. Examples of computations. Computations on ASTs Aspect oriented programming

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1 Semantic analysis Compiler Construction source code scanner semantic analysis Computations on ASTs Aspect oriented programming tokens AST with attributes Lennart Andersson parser code generation Revision parse tree 2012 AST builder AST Compiler Construction 2012 F06-1 Examples of computations Compiler Construction 2012 F06-2 An expression evaluator Name analysis: find the declaration of an identifier Type analysis: compute the type of an expression Expression evaluation: compute the value of a constant expression Code generation: compute an intermediate code representation of a program Unparsing: compute a text representation of a program Abstract grammar abstract Expr; abstract BinExpr : Expr ::= Left:Expr Right:Expr; Add : BinExpr; Sub : BinExpr; IntExpr : Expr ::= <INT:String>; Compiler Construction 2012 F06-3 Compiler Construction 2012 F06-4

2 Evaluator implementation abstract class Expr { abstract int value(); abstract class BinExpr extends Expr { class Add extends BinExpr { class Sub extends BinExpr { Compiler Construction 2012 F06-5 The Interpreter design pattern An unparser abstract Stmt; IfStmt : Stmt ::= Cond:Expr Then:Stmt [Else:Stmt]; abstract class Stmt { abstract void unparse(printstream s, String indent); class IfStmt extends Stmt { void unparse(printstream s, String indent) { s.print(indent); s.print("if "); getcond().unparse(s, indent); s.println(" then "); getthen().unparse(s, indent + " "); if (haselse()) { s.println(" else "); getelse().unparse(s, indent + " "); Compiler Construction 2012 F06-7 Interpreter pattern Intent Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. Example Computer in the OMD course (eda061/edaf10) Expression * interpret(context) TerminalExpression NonTerminalExpression interpret(context) interpret(context) Compiler Construction 2012 F06-8 Compiler Construction 2012 F06-9

3 Example applications The expression evaluator value() plays the role of the interpret method no Context was needed (but would be needed if we had a language with identifiers) Expr plays the role of the abstract Expression IntExpr plays the role of TerminalExpression Add and Sub play the role of NonTerminalExpression The unparser unparse() plays the role of the interpret method Context consists of two parameters: (PrintStream s, String indent) Stmt and Expr play the role of abstract Expressions IfStmt plays the role of NonTerminalExpression... Count the number of identifiers Abstract grammar abstract Stmt; IfStmt : Stmt ::= Cond:Expr Then:Stmt [Else:Stmt];... abstract Expr; abstract BinExpr : Expr ::= Left:Expr Right:Expr; Add : BinExpr; Sub : BinExpr; IntExpr : Expr ::= <INT:String>; IdExpr : Expr ::= <ID:String>;... Compiler Construction 2012 F06-10 Implementation of the counter Compiler Construction 2012 F06-11 The Composite design pattern class ASTNode { int countids() { int count = 0; for (int k = 0; k < getnumchild(); k++) { count += getchild(k).countids(); return count; class IdExp extends Expr { int countids() { return 1; Intent Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. Example Any recursive data type. Compiler Construction 2012 F06-12 Compiler Construction 2012 F06-13

4 The Composite design pattern Example applications Leaf operation(context) Component operation(context) * children Node operation(context) for all g in children g.operation(context) The counter countids() plays the role of the operation method ASTNode plays the role of Component IdExp plays the role of Leaf Other classes play the role of Node. Rather than implementing count() in each of these classes, they inherit a default implementation of ASTNode. Other typical applications Graphical objects that can be grouped hierarchically Line, Rectangle, Text, etc. play the role of Leaf Group plays the role of Node paint(graphics) plays the role of an operation method Compiler Construction 2012 F06-14 Modularization How can new functionality be added to the AST classes? methods, instance variables,... Modify the AST classes? not modular risky to modify generated code what happens if we need to regenerate the AST classes? Modular techniques Intertype declarations (Static Aspect-Oriented Programming) Visitor pattern Compiler Construction 2012 F06-16 Compiler Construction 2012 F06-15 Aspect-Oriented Programming Cross-cutting concerns One aspect concerns many classes The aspect cross-cuts the normal language constructs and leads to tangled code AOP techniques aspect constructs can modularize cross-cutting code, thereby avoiding tangled code A special kind of compiler, an aspect weaver, weaves together aspect code and ordinary code during compilation AOP systems AOP systems AspectJ, (from Xerox PARC) Hyper/J, (IBM) AOSD homepage: Compiler Construction 2012 F06-17

5 Example of Tangled Code Implementation of expression evaluator and unparser abstract class Expr { abstract int value(); abstract void unparse(stream s, String indent); abstract class BinExpr extends Expr { Expr getleft() {... Expr getright() {... class Add extends BinExpr { int value() {... void unparse(stream s, String indent) {... String getint() {... int value() {... void unparse(stream s, String indent) {... Compiler Construction 2012 F06-18 Example Intertype declarations (Static AOP) Partial class definitions can be written in different modules A preprocessor can weave together the code to complete (tangled) classes that are compiled by an ordinary compiler. Compiler Construction 2012 F06-19 Extract functionality class Add extends Expr { expr1.print(); expr2.print(); System.out.print(value); class Add extends Expr { aspect Print { void Add.print() { expr1.print(); expr2.print(); void IntExpr.print() { System.out.print(value); Compiler Construction 2012 F06-20 Compiler Construction 2012 F06-21

6 More functionality Weave class Add extends Expr { aspect Value { int Add.value() { int n1 = expr1.value(); int n2 = expr2.value(); return n1+n2: int IntExpr.value() { return value; java -jar jastadd2.jar Expr.ast produces class Add extends Expr { expr1.print(); expr2.print(); int value() { int n1 = expr1.value(); int n2 = expr2.value(); return n1+n2: Print.jadd Value.jadd Compiler Construction 2012 F06-22 The JastAdd system Compiler Construction 2012 F06-23 Modularizing our example Typed AST Generates AST classes with typed access methods from an abstract grammar Intertype declarations partial class definitions for AST classes are written in.jadd files. JastAdd weaves in these partial definitions into the generated AST classes Rewritable Reference Attribute Grammars Declarative computations using attributes and equations Declarative transformations of the AST Guest lecture by Emma Söderberg. Expr.ast abstract Expr; BinExpr: Expr ::= Expr Expr; Add: BinExpr; IntExpr: Expr; jastadd Expr.java Print.jadd Add.java Value.jadd aspect Value { abstract int Expr.value(); int Add.value() {... int IntExpr.value() {... aspect Print { abstract void Expr.print(); void Add.print() {... void IntExpr.print() {... Compiler Construction 2012 F06-24 Compiler Construction 2012 F06-25

7 JastAdd aspect for the expression evaluator aspect Value { abstract int Expr.value(); int Add.value() { return getleft().value() + getright().value(); int Sub.value() { return getleft().value() - getright().value(); What parts of AST classes can be factored out to.jadd files? Methods Instance variables implements clauses import clauses int IntExpr.value() { return String.parseInt(getINT()); Compiler Construction 2012 F06-26 Example (instance variable) Compiler Construction 2012 F06-27 Visitors Add an integer representation of INT values (in addition to the String representation) aspect IntValue { int Expr.value;... How to modularize in Java (or any other OO language) if we do not have access to AOP mechanisms? Compiler Construction 2012 F06-28 Compiler Construction 2012 F06-29

8 Example How can we factor out the code for print()? class Add extends Expr { expr1.print(); expr2.print(); System.out.print(value); Move functionality class Add extends Expr { class Visitor { void visit(add node) { node.expr1.print(); expr1.print(); node.expr2.print(); expr2.print(); void visit(intexpr node){ System.out.print( node.value); System.out.print(value); Compiler Construction 2012 F06-30 Missing functionality Compiler Construction 2012 F06-31 Delegate class Add extends Expr { class Visitor { void visit(add node) { node.expr1.print(); node.expr2.print(); void visit(intexpr node){ System.out.print( node.value); class Add extends Expr { class Visitor { void visit(add node) { void accept(visitor v) { node.expr1.accept(this); v.visit(this); node.expr2.accept(this); void visit(intexpr node){ System.out.print( void accept(visitor v) { node.value); v.visit(this); Compiler Construction 2012 F06-32 Compiler Construction 2012 F06-33

9 Why not call visit directly? Generalise class Add extends Expr { class Visitor { void visit(add node) { void accept(visitor v) { visit(node.expr1); v.visit(this); visit(node.expr2); void visit(intexpr node){ System.out.print( void accept(visitor v) { node.value); v.visit(this); class Add extends Expr { interface Visitor { void visit(add node); void accept(visitor v) { void visit(intexpr node); v.visit(this); class PrintVisitor implements Visitor { void visit(add node) { node.expr1.accept(this); void accept(visitor v) { v.visit(this); node.expr1.accept(this);... Compiler Construction 2012 F06-34 Generalise with parameter and return interface Visitor { Object visit(add node, Object data); Object visit(intexpr node, Object data); Object accept(visitor v, Object data) { return v.visit(this, data); class PrintVisitor extends Visitor { Object visit(intexpr node, Object data) { System.out.print(node.value); return null; Compiler Construction 2012 F06-36 Compiler Construction 2012 F06-35 Another visitor class ValueVisitor extends Visitor { Object visit(add node, Object data) { int n1 = (Integer) node.expr1.accept(this, data); int n2 = (Integer) node.expr2.accept(this, data); return new Integer(n1+n2); Object visit(intexpr node, Object data) { return new Integer(node.value); Expr expr = new Add(... ); expr.accept(new PrintVisitor(), null); int value = expr.accept(new ValueVisitor(), null); Compiler Construction 2012 F06-37

10 The Visitor Pattern Intent Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. Sketch Expr Sub class Add accept(...) each AST class has an Object accept(visitor, Object) method ValueVistor.java delegates computation class ValueVisitor implements Visitor { Object visit(add, Object) {... Object visit(sub, Object) {... Visitor.java interface Visitor { Object visit(add, Object); Object visit(sub, Object); //one visit method for each AST class UnparseVisitor.java class UnparseVisitor implements Visitor { Object visit(add, Object) {... Object visit(sub, Object) {... Compiler Construction 2012 F06-38 Interface Visitor interface Visitor { Object visit(add node, Object data); Object visit(sub node, Object data); Object visit(intexpr node, Object data);... The visit method is overloaded for different AST argument types Each method returns an untyped object Each method has an untyped argument (data) Compiler Construction 2012 F06-40 Compiler Construction 2012 F06-39 Visitor support in AST nodes Method accept that delegates the computation to a Visitor abstract class Expr { abstract Object accept(visitor v, Object data); abstract class BinExpr extends Expr { Object accept(visitor v, Object data) { return v.visit(this, data); class Add extends BinExpr { Object accept(visitor v, Object data) { return v.visit(this, data); Object accept(visitor v, Object data) { return v.visit(this, data); Compiler Construction 2012 F06-41

11 The evaluator as a visitor To note about this example... class Evaluator implements Visitor { the argument data was not needed for the computation the result (of type int) had to be wrapped in an Integer object Casts are needed in some places. the method value hides implementation detail and makes it easy to call the visitor from a client Example of use Expr e =...; print("the value is "); println(evaluator.value(e)); Compiler Construction 2012 F06-42 The unparser as a visitor Compiler Construction 2012 F06-43 To note about this example... class Unparser implements Visitor { the argument data is used for representing the argument indent and needs to be cast to String before use the original argument PrintStream s is stored in the Visitor object and can be used directly by all visit methods the result value is not used the method unparse hides implementation detail and makes it easy to call the visitor from a client Example of use Program p =...; PrintStream s =...; Unparser.unparse(p, s); Compiler Construction 2012 F06-44 Compiler Construction 2012 F06-45

12 One more example Count the number of identifiers in a program abstract Stmt; IfStmt : Stmt ::= Cond:Expr Then:Stmt [Else:Stmt];... abstract Expr; abstract BinExpr : Expr ::= Left:Expr Right:Expr; Add : BinExpr ::= ; Sub : BinExpr ::= ; Int : Expr ::= <INT:String>; IdExpr : Expr ::= <ID:String>;... How can the Visitor be implemented? Compiler Construction 2012 F06-46 CountIdentifiers as a visitor TraversingVisitor class TraversingVisitor implements Visitor { Object visit(ifstmt node, Object data) { node.getcond().accept(this, data); node.getthen().accept(this, data); if (node.haselse()) { node.getelse().accept(this, data); Object visit(add node, Object data) { node.getleft().accept(this, data); node.getright().accept(this, data); The code above is independent of the computation and could have been generated from the abstract grammar (but this is currently not done in JJTree or JastAdd). Compiler Construction 2012 F06-47 Intertype declarations vs. Visitor class CountIdentifiers extends TraversingVisitor { Example of use types for arguments and return values separate compilation? pure Java? declar- intertype tions what can be modularized? instance variables, methods, implements clauses arbitrary no preprocessor required no requires additional tools Visitor only methods Object visit(..., Object) (one untyped argument and one result) yes yes Compiler Construction 2012 F06-48 Compiler Construction 2012 F06-49

13 Using the modularization techniques Interpreter, AST Interpretation Unparsing Metrics Semantic Analysis Name analysis connect an identifier to its declaration Type analysis? compute the type of an expression... Code generation Compute the size needed for objects and methods Generate instructions... abstract Stmt; Block: Stmt ::= Stmt*; Assignment:Stmt ::= <ID> Expr; abstract Expr; Add:Expr ::= Left:Expr Right:Expr; IdExpr:Expr ::= <ID>; Compiler Construction 2012 F06-50 Compiler Construction 2012 F06-51 Interpreter, methods abstract class Expr extends ASTNode { abstract int value(map<string, int> map); class Add extends Expr { int value(map<string, int> map) { return getleft().value(map) + getright().value(map); class IdExpr extends Expr { int value(map<string, int> map) { return map.get(getid()); Compiler Construction 2012 F06-52 Interpreter, methods abstract class Stmt extends ASTNode { abstract void execute(map<string, int> map); class Block extends Stmt { void execute(map<string, int> map) { for (int i=0, i>getnumstmt(), i++) { getstmt(i).execute(map); class Assignment extends Stmt { void execute(map<string, int> map) { int value = getexpr().value(map); map.put(getid(), value)); Compiler Construction 2012 F06-53

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